Accelerator pump question

[canadian_fisher]

My '95 Merc 100 is having tough time accelerating between 1800-2600 RPMs.
Below 1800, she idles good but not great.
Above 2600, she runs real nice right upto 5200 RPM at WOT.
To the best of my knowledge, I have a 2 + 2 ignition system that engages cylinders 3 & 4 at around 1800 RPM. My best-guess is a fuel related issue, and not electrical. I have tried my best to set the Idle-mixture and timing properly, but still no luck.

So here are the question(s):
(1) What is the purpose of the Accelerator Pump that feeds the Fuel Tee between cylinders 2 and 3?
(2) How do I test if its working?
(3) Anybody experience this flat-spot in the same RPM.

Help is greatly appreciated.

CF

 

[WillyBWright]

Re: Accelerator pump question

You have the same characteristic that everybody else experiences with that motor as well as the 115 and 125 versions. Just consider it an idiosyncracy and avoid that range. There's nothing you can do about it. The accelerator pump supplies the extra fuel to the bottom two cylinders for rapid acceleration. It only affects those two and only if it's rapdly engaged. The upper cylinders get the extra jolt from normal jetting. It's strictly a carburetion thing. It sparks on all four all the time. Do a Search and you'll find that the subject is discussed frequently.

 

[canadian_fisher]

Re: Accelerator pump question

Hey WillyB,

Call me crazy, but its seems a little bizzare that the Number #1 name in Outboards made a major design flaw in the 2 + 2 ?!?!?
I have been trying to find more information, but not sure on keywords to search on.
Are there any suggestions on how to make it run a little smoother at least?? (ie Octance boost, timing advance / retard, modify jetting, upgrades to anything??.......)

Question:
What are the symptoms when the Accelerator Pump fails?

Thanks for the great information.

CF

 

[canadian_fisher]

Re: Accelerator pump question

UPDATE !!!!!

I couldn't resist and called Mercury Outboard Tech Support here in Canada.
They definately admitted that the 2 + 2 was a bizarre design, but allows larger motors (like my 100HP) to idle and run smoother at lower RPMs.
Also, they suggested that the OEM carb settings for Idle-Mixture are actually 1-1/4 (+/-1/4)turns rather than 1-3/4(+/- 1/4) turns from my Seloc Service Manual.

Well I tried it last weekend dialing it down as suggested , and it WORKED. What a huge diffrerence!!!
My 100HP now gently idles at 750 rpm and I was trolling non-stop for over an hour w/o hiccup, stalling or starvation..........
And we caught a 15-pound Musky......... very cool.

If nobody is aware, the 2 + 2 is a VERY quite motor when both trolling and WOT......... very impressed overall.

Thanks folks........ CF